Commits on Feb 4, 2018

1. Reimplementations of immutable HashSet and HashMap. …

   The reimplementations are based upon Compressed Hash-Array Mapped Prefix-trees (CHAMP), see paper "Optimizing Hash-Array Mapped Tries for Fast and Lean Immutable JVM Collections" by Steindorfer and Vinju (OOPSLA'15) for more details and descriptions of low-level performance optimizations (a pre-print of the paper is available under https://michael.steindorfer.name/publications/oopsla15.pdf). This commit closes scala#192.
   
   The new implementations (i.e., ChampHashSet and ChampHashMap) currently exist next to the previous HashMap and HashSet. By default immutable.Map and immutable.Set now pickup the CHAMP data structures. A JVM flag (-Dstrawman.collection.immutable.useBaseline=true) allows to switch back to the previous HashSet and HashMap implementations for testing. Note, the flag and the previous HashSet and HashMap implementations will be removed in the final version of collection-strawman, but for the time being they remain to support comparing the different trade-offs and performance characteristics of the current and the new data structures.
   
   Preliminary performance numbers of the new CHAMP data structures were presented in issue scala#192. Overall one can summarize that the CHAMP data structures significantly lower memory footprints and significantly improve all iteration-based operations and equality checks. Basic operations such as lookup, insertion, and deletion may slow down. The current state of the reimplementation does not optimize for hash-collisions yet.
   
   Note that the CHAMP design / implementation differs from the previous immutable hashed data structures by not memoizing the hash codes of the individual elements (which may change the performance of certain workloads). If necessary, CHAMP's design allows to modularly add memoized hash codes of the individual elements (at the expense of some memory savings). Details are discussed in the paper mentioned above.

   

   msteindorfer committed Feb 4, 2018
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